1. Field of the Invention
The present invention relates generally to composite materials that contain a thermoplastic-toughened cyanate ester resin as the resin matrix. These high-strength composites are suitable for use as primary structures in aircraft and other load-bearing applications. The present invention is directed to the properties of such composite materials when they are burned. In particular the invention involves providing such thermoplastic-toughened cyanate ester composites that have low heat release properties and short self-extinguishing times.
2. Description of Related Art
Cyanate ester resins that are reinforced with a fibrous material, such as glass or carbon fiber, are used in a wide variety of situations where high structural strength and low-weight are required. Composite materials that use a high performance cyanate ester resin matrix are especially popular in the aerospace industry where weight and structural strength are important engineering and design considerations. High performance cyanate ester resins typically include one or more thermoplastic materials that provide “toughening” of the cyanate ester resin. Although such high performance cyanate ester resin composite materials are desirable because of their relatively high strength to weight ratio, they do present some specific issues with respect to heat release, flammability and other burn properties.
A major goal in developing formulations for high performance cyanate ester resin composites is to limit the amount of heat that is released during burning, while at the same time not reducing the structural strength of the cured composite part. This is especially important for primary structures and parts that are located in the interior areas of aircraft. It is also important that any attempt to reduce heat release does not adversely affect properties of the uncured cyanate ester resin, such as tack and viscosity. The tack and viscosity of the uncured resin are especially important when the cyanate ester resin is used to make prepreg, which is a common intermediate material used in the fabrication of aerospace parts.
Heat release requirements for composite materials, which are used in the interior of aircraft, are set forth in the Federal Aviation Administration (FAA) Aircraft Materials Fire Test Handbook (14 C.F.R 25.853(d), Appendix F, Part 1) and BOEING Specification Support Standard BSS-7322. The total rate of heat release during combustion of a test sample is measured as well as the peak heat release rate during the burning process. The standard test used to determine the heat release requirements for composite materials exposed to radiant heat is the Ohio State University (OSU) heat release test. It is desirable to provide cyanate ester resin composites that have an average OSU total heat release rate at two minutes of less than 65 kilowatt-minutes per square meter (kw-min/m2) and a peak OSU heat release rate that is less than 65 kw-min/m2. These two OSU values are the minimum requirements set by 14 C.F.R. 25.853(d), Appendix F Part IV, for interior aircraft parts made from composite materials.
Resistance to surface flammability is also an important area of concern for high performance cyanate ester resin composites. It is important that a cyanate ester resin composite part, which is on fire, be able to self-extinguish once the source of heat and/or flame is removed. The ability to self-extinguish is an especially important consideration for primary structures and parts located in the interior areas of aircraft. It is also a significant goal of cyanate ester resin formulators to develop cyanate ester resins that are used to make composites which self-extinguish in as short a time period as possible, while at the same time keeping structural strength of the finished composite part at the levels needed for aerospace applications. The same requirement that the tack and viscosity of the uncured cyanate ester resin not be adversely affected applies with respect to attempts to formulate cyanate ester resins with short self-extinguishing times.
The United States Federal Aviation Administration has established regulations and requirements for fire resistance of aircraft interior parts and materials. These requirements are set forth in 14 C.F.R. 25.853(a). One requirement is that the composite material be able to self-extinguish once the flame source is removed. The test procedure for measuring the self-extinguishing time for cyanate ester resins are also set forth in the FAA Aircraft Materials Fire Test Handbook (FAR 25.853, Appendix F, Part 1) and in BOEING Specification Support Standard BSS-7230 (Revision H), which is recognized in the aerospace industry as a standard test method. It would be desirable to provide high performance cyanate ester resin composites where the self-extinguishing times are as short as possible and at least below 15 seconds. A 15 second self-extinguishing time is the maximum allowed for composite aircraft parts pursuant to 14 C.F.R. 25.853(a).